1. Field of the Invention
The present invention relates to coils to be provided in rectifier circuits, noise eliminating circuits, resonance circuits, etc. for use in various AC devices, and a process for fabricating the coils.
2. Description of the Related Art
Conventionally known is a coil device of the toroidal type, which comprises an air-core coil 81 fitted around a bobbin 10, as shown in FIG. 11. The air-core coil 81 is fabricated, for example, by winding a conductor around an outer surface of a wire winding jig (not shown) in the order indicated by the numerals of 1 to 29 as shown in the drawing. First the conductor is wound around the outer surface of the jig in the order of 1 to 10 to form a first layer 82, thereafter the conductor is wound around the outer surface of the first layer 82 in the order of 11 to 19 to form a second layer 83, and finally the conductor is wound around the outer surface of the second layer 83 in the order of 20 to 29 to form a third layer 84, to thereby fabricate the air-core coil 81 having three layers.
With the air-core coil 81 shown in FIG. 11, however, the first layer 82, the second layer 83 and the third layer 84 are lapped over as connected to each other in series. This results in the appearance of a stray capacity between each pair of turns of the conductor adjacent to each other axially of the coil and the appearance of a stray capacity between each pair of turns of the conductor lapped over in a direction orthogonal to an axis of the coil, as shown in FIG. 12. In this case the number 1 turn in the first layer 82 and the number 19 turn in the second layer 83 are lapped over each other, and the number 11 turn in the second layer 83 and the number 29 turn in the third layer 84 are lapped over each other, thus rendering high a potential difference between the turns lapped over each other, i.e., voltage across the layers, as shown in FIG. 12. This gives rise to the problem of the voltage resistance of the air-core coil 81. Furthermore there is also the problem of impaired frequency characteristics of the air-core coil 81 due to the increased stray capacity.
The present applicant has proposed the process shown in FIGS. 13(a) and 13(b) for fabricating a coil device which comprises a coil fitted around a core (see the publication of JP-A No. 2000-277337). According to this fabrication process, a coil device as shown in FIG. 13(b) is fabricated by inserting one side portion of an air-core coil 8 into a center hole 70 of a C-shaped core 7 through a gap portion 71 thereof as shown in FIG. 13(a) and fitting the coil 8 around the core 7. With this fabrication process, the air-core coil 8 separated from the core 7 is made, and the coil 8 is thereafter fitted around the core 7 to complete the coil device. The process is therefore simplified by eliminating the need to wind a wire around the core 7 and making the air-core coil 8 automatically.
In fabricating the conventional coil device shown in FIGS. 13(a) and 13(b), a rectangular conductor or trapezoidal conductor can be used as the conductor of the air-core coil in order to increase the ratio of the sectional area of the turns of conductor 9 passing through the center hole 70 of the core 7, to the total area of the center hole 70, i.e., the space factor of the conductor 9. When having the same cross sectional area as a round conductor, the rectangular conductor and trapezoidal conductor have a short side which is smaller than the diameter of the round conductor, so that an increased number of turns of conductor can then be accommodated in the center hole 70 of the core 7, hence a higher space factor. However, the rectangular or trapezoidal conductor has the problem of being more expensive than the round conductor.
Another process for fabricating a coil device of higher space factor is known which comprises winding a conductor 9 around a core 7 in the order indicated by the numerals of 1 to 13 in FIG. 14(a), and thereafter winding the conductor 9 around the core 7 in the order indicated by the numerals of 14 to 23 in FIG. 14(b) so as to provide one coil layer on the outer peripheral side of the core 7 and two coil layers on the inner peripheral side of the core 7. An increased number of turns of conductor can then be accommodated in the center hole 70 of the core 7 to result in a higher space factor. The conductor 9 is nevertheless difficult to wind around the core 7 automatically and must be wound by manual work, which involves the problem of low production efficiency.
Accordingly, an object of the present invention is to provide an air-core coil which has a lower voltage across the layers than conventionally and improved frequency characteristics and which can achieve a high space factor without using a rectangular or trapezoidal conductor, and a process for fabricating the air-core coil which process can be practiced automatically.